1. Field of the Invention
The present invention relates to cases to be used for the storage or transportation of memory disks such as compact disks and magnetic disks, especially hard disks, and particularly to cases for memory disks having excellent antistatic properties, capable of preventing the generation of static electricity which may be caused during the storage or transportation of memory disks.
2. Background Art
Heretofore, crystalline propylene thermoplastic resins have been used to make cases or containers to be employed for storage or transportation in various fields. This is because such resins are economical, excellent in molding properties and durability, and also light in weight.
However, crystalline propylene thermoplastic resins have high electrical resistance, and are not conductive, so that cases made from these resins easily generate static electricity and undergo static electrification due to friction, impact, vibration or the like caused during transportation. Various troubles are thus caused.
In particular, in the field of cases for electronic equipments, there have been problems that ICs, LSIs and the like placed in the cases are broken or damaged by static electricity generated due to friction, impact, vibration or the like while the cases are being transported. For this reason, it has been a serious subject to impart antistatic properties to such cases.
In particular, with respect to cases to be used for the transportation of memory disks, there have been made studies on techniques relating to the structure and shape of a case which can hold or bind memory disks by connecting their edges to the case so as to prevent the movement of the memory disks, thereby protecting the memory disks from damages due to static electricity, or of a case which can be subjected to a processing step with memory disks stored therein (e.g., Japanese Patent Publication No. 55395/1989).
On the other hand, in order to solve the static electricity problem from the material viewpoint, a conductive filler such as carbon black, or an antistatic agent has been conventionally incorporated into a propylene resin so as to impart thereto conductivity, thereby obtaining a composite resin having antistatic properties.
Metallic fiber, metallized fiber, carbon black, carbon fiber, graphite, tin oxide, zinc oxide, indium oxide or the like has been used as the conductive filler.
Although such a composite plastic incorporated with a conductive filler can stably and permanently reveal antistatic properties, cases to be used for storage or transportation, made from the composite plastic are heavy in weight because the specific gravity of the filler incorporated is high. Further, some problems other than the problem of static electricity may be newly introduced by staining induced by heavy metallic impurities contained in the above-described conductive fillers, or by dusting caused, depending upon the manner of use, due to the separation of the conductive filler from the composite plastic.
A glycerin-fatty acid ester, an alkyl diethanol amide, a sorbitan-fatty acid ester or the like has been generally used as the antistatic agent to be incorporated into a propylene resin. Such an antistatic agent has a low molecular weight, so that the agent migrates to the surface of the resin and bleeds therefrom after the resin incorporated with the agent is molded. The resin can thus exhibit antistatic properties.
However, the antistatic agent bleeding on the surface of the resin is readily removed by washing or the like. It is therefore difficult for the resin to stably and permanently retain the antistatic properties. Further, a case made from the resin has such a shortcoming that the antistatic agent bleeding on the surface of the resin tends to stain the content of the case, for example, memory disks.
Thus, the above-described conventional propylene resins imparted with antistatic properties are still unsatisfactory for use as cases for the storage or transportation of memory disks.
In order to solve the aforementioned problems, techniques for imparting antistatic properties to propylene resins, in which one of the following hydrophilic polymers is incorporated into a propylene resin, have been proposed recently:
(1) a mixture of a specific polyether ester amide and a modified vinyl copolymer having a carboxyl group, disclosed in Japanese Laid-Open Patent Publication No. 23435/1985; PA1 (2) a comb copolymer of a polymeric monomer obtained by converting carboxyl group at the terminal end of polymethyl methacrylate into methacryloyl group by using glycidyl methacrylate, and an aminoalkyl acrylate or acrylamide, or a quaternized cation-modified product thereof, disclosed in Japanese Laid-Open Patent Publication No. 121717/1987; and PA1 (3) an acrylamide copolymer consisting of ethylene structural unit, acrylate structural unit and acrylamide structural unit, or a polyolefin resin composition comprising the acrylamide copolymer, disclosed in U.S. Pat. No. 5,202,193 and Japanese Laid-Open Patent Publication 126446/1995. PA1 component (A): a crystalline propylene resin having a melt flow rate (MFR) of 1 to 80 g/10 min; PA1 component (B): a thermoplastic elastomer selected from (b.sup.1) olefin copolymer elastomers having a Mooney viscosity (ML1+4 (100.degree. C.)) of 10 to 85, and (b.sup.2) hydrogenated products of a block copolymer of styrene or a derivative thereof and a conjugated diene, containing 15 to 50% by weight of styrene or a derivative thereof, having a weight-average molecular weight of 50,000 to 220,000; and PA1 component (C): an acrylamide copolymer.
Such a propylene resin composition incorporated with any one of the above hydrophilic polymers is light in weight, and can stably and permanently reveal antistatic properties; however, the hydrophilic polymer particles dispersed in the resin composition tend to form streaks in the direction of molding when the resin composition is melt molded because the compatibility between the hydrophilic polymer and the propylene resin is poor. As a result, delamination is caused due to the streakily dispersed hydrophilic polymer particles, existing in the vicinity of the surface of the molded product. The delamination frequently causes such troubles that the appearance of the molded product becomes worse and that the impact resistance of the molded product is impaired. A satisfactory propylene resin case having antistatic properties is thus not available so far.
An object of the present invention is to provide a case for memory disks which can stably and permanently reveal antistatic properties, which is free from the shortcomings in the prior art, such as the separation or uneven dispersion of an antistatic-properties-imparting component, and which has high mechanical strength and lightweight properties.